This invention relates in general to the field of digital circuits and in particular to reducing propagation delay dependence on supply voltage in a digital circuit.
The supply voltage in an integrated circuit (IC) does not remain precisely constant. Instead, the supply voltage fluctuates as a result of noise attributable to the normal operation of the IC. Such noise may be caused, for example, by inductive coupling occurring between the IC and external devices. Typically, supply voltage fluctuation is an undesirable occurrence. For example, in analog ICs, a filter may be used to reduce fluctuation in the supply voltage caused by noise. However, for a number of reasons, similar filters may not be suitable for use in digital ICs. Historically, designers, manufacturers, and end users of digital ICs have simply tolerated supply voltage fluctuation caused by noise, despite the fact that supply voltage fluctuation may adversely affect the performance of the IC. In an all-digital phase-locked loop (ADPLL), for example, a delay circuit is typically used to set the frequency of the loop. Since the propagation delay of a signal through the delay circuit may vary over a range of supply voltages, supply voltage fluctuation attributable to noise may cause the frequency of the ADPLL to fluctuate, a phenomenon commonly known as xe2x80x9cjitter,xe2x80x9d thereby adversely affecting the performance of the ADPLL and the digital IC containing the ADPLL.
According to the present invention, disadvantages and problems associated with propagation delay dependence on supply voltage in digital circuits are substantially reduced or eliminated.
According to one embodiment of the present invention, a digital circuit element has a propagation delay that is substantially constant over a range of supply voltages applied to the digital circuit element.
In another embodiment of the present invention, a digital circuit element having a propagation delay that is substantially constant over a range of supply voltages applied to the digital circuit element includes an input node, an output node, and at least one gate coupling the input node and the output node. A plurality of possible voltage transition curves is associated with a corresponding change of a first voltage at the input node over time, each voltage transition curve being determined by a corresponding supply voltage and the curves intersecting within a relatively narrow range of voltages. The gate is operable to change a second voltage at the output node in response to the first voltage reaching a threshold voltage of the gate, and the threshold voltage is set within the relatively narrow range of voltages in which the voltage transition curves intersect in order to reduce the dependence of the propagation delay on the supply voltage.
In yet another embodiment, a digital circuit element having a propagation delay that is substantially constant over a range of supply voltages applied to the digital circuit element includes an input node, an output node, and at least one gate coupling the input node and the output node. The gate is operable to change a first voltage at the output node in response to a second voltage at the input node reaching a threshold voltage of the gate, and the threshold voltage of the gate is set such that a delay separating an initial change in the first voltage from an initial change in the second voltage is substantially constant over a range of supply voltages applied to the digital circuit element.
The present invention provides a number of important technical advantages over previous digital circuit elements. Properly setting the gate threshold voltage according to the present invention allows for the design, manufacture, and use of delay elements providing propagation delay that is substantially constant over a range of supply voltages, reducing or eliminating the necessity of filtering or making other changes to the supply voltage. Accordingly, jitter in ADPLLs and other digital circuits may be reduced, and the frequency of these digital circuits may be more precisely controlled, in order to improve performance.
Digital circuits incorporating one or more of these or other technical advantages are well suited for use in modern digital systems. Other technical advantages are readily apparent to those skilled in the art from the following figures, descriptions, and claims.